1. Field of the Invention
The present invention relates to integrated optical waveguides and more particularly to a Luneburg lens provided on an optical waveguide and having a protective coating.
2. Description of the Prior Art
Integrated optics is an attractive approach for providing new devices for signal processing such as scanners, deflectors, modulators, switches, RF spectrum analyzers, convolvers, correlators, multiplexers and demultiplexers, because of the potential for very high performance operation and high speed processing using optical processing principles in structures which can be very small and rigid in planar configuration, and which can be manufactured by batch fabrication techniques.
Such devices need thin-film waveguide lenses to control the shape of the guided beam for imaging, spatial filtering, focusing and Fourier analysis. The lenses for these applications must have high efficiency, high performance and high stability. Further, lens accuracy, meaning that the focal length of the lens shape is accurate enough to satisfy design specifications, is essential for more precise applications such as where a well-collimated guided beam or sufficiently small beam spot size is needed.
One conventional type of integrated optical lens is the Luneburg lens. This lens is a classical index-graded lens having a circularly symmetric refractive index distribution which preferably focuses the arc of a fixed circle onto the arc of a second fixed circle. It is fabricated by sputtering or evaporating the lens material onto the waveguide surface through a circular mask with shaped edges. S. K. Yao, et al., Guided-Wave Optical Thin-Film Luneburg Lenses: Fabrication Technique and Properties, Appl. Optics, 18, 4067 (1979).
In devices requiring a high refractive index Luneburg lens, chalcogenide glasses have been found particularly suitable. However a lens formed from a chalcogenide glass may have the disadvantage that its optical characteristics may not be stable due to the adverse effects of light and/or moisture.